Production of formaldehyde from trithiane



United States Patent 3,420,888 PRODUCTION OF FORMALDEHYDE FROM TRITHIANEAlbert Schiipf, Hering, Erhard Siggel, Seckmauern, and Gerhard Meyer,Obernburg, Germany, assignors to Vereinigte Glanzstoif-Fabriken AG,Wuppertal-Elberfeld, Germany No Drawing. Filed Apr. 11, 1966, Ser. No.541,512

Claims priority, application Girmany, Apr. 23, 1965,

US. Cl. 260-606 Claims Int. Cl. C07c 45/00; C07c 47/04 and can beprepared in a known manner by reacting commercial formaldehyde withhydrogen sulfide gas in an acid solution, e.g. hydrochloric acid. (See,for example, the disclosure by Best and Constable in Organic Syntheses,vol. 16, pp. 81-83 (1936).) It is also known that trithiane accumulatesas a precipitate during the spinning of viscose rayon where the acidspin bath contains formaldehyde. Rather than simply discarding thistrithiane by-product in a wasteful manner, it would be desirable to workup the spin bath by separating the precipitated trithiane and thenconverting it into a useful product, especially a product such asformaldehyde which could against be used for spinning viscose filamentsin the production of rayon.

It is known that formaldehyde can be produced from trithiane by reactingthe trithiane with silver sulfate and calcium oxide in the presence ofoxygen or air at elevated temperatures. This process is very expensive,particularly due to the high price of the silver compound but alsobecause silver sulfide is formed in the reaction with trithiane and itis possible to recover silver sulfate therefrom only with considerable'difiiculty, i.e. by using several reaction stages for the conversion ofthe silver sulfide into silver sulfate. This known process therefore hasno practical significance or commercial advantage, especially in thosecases in which the trithiane merely accrues as an undesirable by-productand must be transformed into formaldehyde in an economically productivemanner. Furthermore, the known process gives relatively low yields offormaldehyde, even when carefully regenerating silver sulfate from thesilver sulfide reaction product.

One object of the present invention is to provide a process for theproduction of formaldehyde from trithiane which can be carried out toobtain good yields in a relatively economical manner.

Another object of the invention is to provide a process for theregeneration of formaldehyde from trithiane initially obtained as aprecipitate in the spinning bath used for the production of viscoserayon filaments.

Still another object of the invention is to provide a continuous methodof converting trithiane into formaldehyde by reaction with substanceswhich are relatively inexpensive and/ or easily regenerated for reuse inthe process.

These and other objects and advantages of the inven- 3,420,888 PatentedJan. 7, 1969 tion will become more apparent upon consideration of thefollowing detailed description.

It has been found, in accordance with the present invention, that verypractical results can be achieved in the production of formaldehyde fromtrithiane by contacting the trithiane with CuO and air, preferably withair or a similar oxygen-containing gas in admixture with steam, at atemperature between about C. and 250 C. and condensing formaldehyde fromthe efiluent gaseous reaction product. The process is preferably carriedout at a temperature between about 210 C. and 235 C., and although thepressure is not a critical factor, the reaction is most convenientlycarried out at approximately atmospheric pressure, i.e. normal pressureof about one atmosphere.

The copper-II-oxide (CuO) is introduced for intimate contact with thetrithiane in accordance with the invention as a finely divided orpulverulent material or else it can be precipitated or applied onto aninert carrier such as aluminum oxide. While the amount of the copper-II-oxide can vary over a broad range, e.g. in a weight ratio of CuO totrithiane of about 1:1 to 6:1, it is preferable to employ thecopper-II-oxide in a slight excess over that which would betheoretically required if all of the oxygen in this copper-oxide weresubstituted for the sulfur of the trithiane. Thus, the preferred ratioby weight of CuO to trithiane is about 2:1 to 4:1. This ratio by weightis taken with reference to the initial materials since at least part ofthe CuO is converted into CuS during the course of the reaction.Although it would be possible to use a much larger excess of CuO, itwill be apparent that this would not ordinarily be economical.

The reaction time is not critical, but it is dependent upon the reactiontemperature and the extent to which the CuO and trithiane are broughtinto intimate contact with each other and with the oxygen-containing gassuch as air. Thus, the reaction proceeds most rapidly at highertemperatures and under conditions assuring the most intimate contact ofthe reactant materials. It is therefore preferable to work with aCuO-trithiane reaction mixture which is extended into a relatively thinlayer of material in the reaction zone, e.g. by channeling the trithianethrough a reaction zone packed with a carrier supported copper-II-oxideor by conducting an intimate mixture of the trithiane andcopper-II-oxide in a relatively thin layer through the reaction zone.However, relatively good results are also achieved without providing asmall layer thickness and without thorough mixing. For example, abouttwo hours have been required for completion of the reaction in tests inwhich the CuO-trithiane mixture is simply placed in a reaction vesseland the reaction is carried out at a temperature of 190 C. withoutsubstantial mixing of the reactant materials. By comparison, whenworking with a smaller layer thickness or more intimate contact of thereactants and with better mixing and more rapid and thorough heating,then the reaction time at 230 C. can be reduced to a period of about twominutes.

It is generally necessary to carry out the reaction within theprescribed temperature range of 190 to 250 C., optimum results beingachieved within the range of about 210 to 235 C. At temperatures below190 C., an excessively long period of time is required to complete thereaction, while at temperatures over 250 C. the formaldehyde product isfurther oxidized so rapidly as to cause a decrease in yield. Theretention time of the formaldehyde product in the reaction zone alsoinfluences the extent to which the formaldehyde is oxidized into formicacid and other secondary by-products. Therefore, some care must beexercised in order to avoid excessive oxidation of the desiredformaldehyde product. Nevertheless,

when working within the scope of the invention, the most favorableconditions can be readily determined by simple preliminary tests. Theformaldehyde product can be obtained by the process according to theinvention in yields of up to about 60%, with reference to the initialamount of trithiane induced into the reaction,

It is especially advantageous to carry out the reaction in a continuousoperation wherein the copper-II-oxide and trithiane are conductedcontinuously through the reaction zone and maintained in intimatecontact with each other and with a stream of air or a mixture of air andsteam. For example, one can utilize the working principle of a singlescrew conveyor consisting of a double jacketed tube equipped with ascrew or worm to convey the reactant materials therethrough. Thisjacketed tube is heated along its entire length by means of a suitablefluid heat-exchange medium, thereby assuring a constant temperature inthe reaction zone. The copper-II-oxide and trithiane are initially mixednad introduced into the reaction tube by means of a conventional feedingor dosing device, and the mixing of these materials is further promotedin the reaction tube by the screw conveyor. The retention time of thereaction mixture can be regulated by controlling the number ofrevolutions of the worm or screw per unit time, i.e. by controlling theturning rate of the screw. The air or other oxygen-containing gas can beintroduced into the screw conveyor for passage therethrough, and theformaldehyde product is then separated with the stream of air at theoutlet end of the reaction zone and condensed by cooling the effluentgas stream, e.g. in an air-cooled condenser.

When the reaction is carried out in this tubular reactor equipped with ascrew conveyor, both the copper-II-oxide and trithiane are carriedconcurrently with each other and it is also preferable to conduct thestream of air or other oxygen-containing gas in the same direction,However, it is also possible to conduct the trithiane 1 incountercurrent flow to the copper-II-oxide, particularly where thelatter is supported on an inert carrier substance such as aluminumoxide. In this case, the carrier supported CuO can flow continuously inone direction through the reaction zone while the trithiane is carriedby the stream of air or similar oxygen-containing gas in the oppositedirection. A CuO-CuS mixture is then removed from one end of thereaction zone and the formaldehyde is removed and condensed from theefliuent gas at the other end of the reaction zone.

Regardless of the exact manner in which the process of the invention iscarried out, the CuO-CuS mixture which is obtained after completion ofthe reaction can be very easily treated in order to regenerate CuO whichcan then be used again for further reaction with fresh trithiane. Thisregenerating treatment is merely a simple roasting of a copper sulfidein the mixture so that it is converted by oxidation back into CuO. Thisroasting can be carried out in situ after the reaction with trithiane iscompleted, or where the process is carried out continuously, the CuO-CuSmixture can be concurrently roasted in a separate continuous reactionstage and then recycled for use in the conversion of trithiane intoformaldehyde.

It is a particular advantage of the invention that the copper-II-oxidecan be reused many times in this manner without any substantialreduction of the formaldehyde yield. In addition, the roasting of thecopper sulfide for conversion into copper-II-oxide is a very well knownsingle step reaction which is relatively inexpensive and which permitsone to avoid a multi-stage regeneration of a metal salt.

The process of the invention is further illustrated by the followingexamples, but it will be understood that the invention is not limited tothese examples.

1 In gaseous form.

4 Example 1 5 grams of trithiane are finely ground with 10 grams of CuOand heated for a period of 2 hours in a 250 ml. two-necked flask whilepassing air through the flask at a temperature of l-200 C. The efliuentgas stream is passed through an attached cooling tube in which there areseparated 1.47 grams of paraformaldehyde. This corresponds to a yield of45% Example 2 5 grams of trithiane are thoroughly mixed with 15 grams ofCuO and heated for a period of one hour in a flask as in Example 1 withpassage of air therethrough at 230 C. In the cooling tube, 1.65 grams ofparaformaldehyde are separated, corresopnding to a yield of 51%.

- Example 3 2 grams of trithiane are forced with a strong air streamcontaining steam over a packing or filling arranged in a reaction tubeconsisting of 7 grams of CuO on a carrier of 35 grams of A1 0 which ismaintained at a temperature of 220 C. The steam is preheated to 210 C.and introduced at the rate of 275 grams/hour in admixture with 60liters/hour of air. A condensate is separated at the outlet end of thetube which contains 0.73 gram of formaldehyde. With reference to theinitial 2 grams of trithiane, the resulting yield is 56%.

Example 4 1 part by weight of trithiane is finely ground with 3 parts byweight of CuO and led into a reaction tube heated by indirect heatexchange to 230 C. The reaction mixture is moved continuosly forward inthe tube by means of a screw conveyor. The number of revolutions of thescrew is determined in such a manner that the average retention time ofthe reaction mass in the heated tube amounts to about two minutes. Atthe same time, a stream of air preheated to 230 C. is led into the tubefor concurrent flow and contact with the reaction mixture. At thedischarge end of the reaction tube, the paraformaldehyde is removed withthe air and is condensed and collected in an air cooler. The yieldamounts to 60% with reference to the initial trithiane.

The resulting CuS-CuO mixture removed at the discharge end of the tubeis converted back into CuO through a simple roasting, and can thenreintroduced with fresh trithiane. In the reutilization of thisregenerated CuO in the above described apparatus, a yield of 56%formaldehyde is obtained.

COMPARATIVE EXAMPLE 1 part by weight of trithiane is intimately mixedwith 2 parts by weight of Ag SO and 1 part by weight of CaO, and isconveyed with mixing through a reaction tube heated to 230 C. asdescribed in Example 4. The yield of paraformaldehyde amounts to 33 Ifone attempts to regenerate the Ag SO /CaO-mixture from the dischargedsolids in the same manner as described in Example 4, i.e. by a simpleroasting step, then in the reutilization of the roasted product, thereis obtained a yield of only about 3.6% formaldehyde. In order to achieveabout the same yield of formaldehyde as described above, i.e. 33%, thesilver catalyst must be converted back into Ag SO after thoroughlyroasting the sulfur in known manner, i.e. by further treatment withsulfuric acid and evaporation.

It will be apparent from the foregoing examples that the process of thepresent invention not only permits a substantial increase in the yieldof formaldehyde but also avoids a costly and diflicult regeneration ofthe metal catalyst. The formaldehyde product is obtained from theprocess in the form of a good commercial grade of paraformaldehyde andcan therefore be used directly in the spinning bath for the productionof viscose rayon filaments. Furthermore, the process of the inventioncan be readily adapted to continuous operation, including theregeneration of the copper oxide from copper sulfide formed during thereaction. For these reasons, the novel process of the invention is quitesuitable for commercial exploitation, and variations or modifications ofthe process can be readily made by one skilled in this art withoutdeparting from the spirit or scope of the invention as claimedhereinbelow.

The invention is hereby claimed as follows:

1. A process for the production of formaldehyde from trithiane whichcomprises contacting said trithiane with CuO and air at a temperaturebetween about 190 C. and 250 C. and condensing formaldehyde from theefiluent gaseous reaction product.

2. A process as claimed in claim 1 wherein said temperature is betweenabout 210 C. and 230 C.

3. A process as 'claimed in claim 1 wherein said air is admixed withsteam for contact with said trithiane.

4. A process as claimed in claim 1 wherein the ratio by weight of CuO totrithiane is about 1:1 to 6:1.

5. A process as claimed in claim 1 wherein said CuO is deposited on aninert carrier.

6. A process as claimed in claim 1 wherein a mixture of trithiane andCuO is conducted continuously through a reaction zone maintained at atemperature between 6 about C. and 250 C. together with a preheatedstream of air.

7. A process as claimed in claim 6 wherein said reaction zone ismaintained at a temperature between about 210 C. and 230 C.

8. A process as claimed in claim 6 wherein said stream of air is admixedwith steam.

9. A process as claimed in claim 6 wherein the ratio by weight of CuO totrithiane in said mixture is about 2:1 to 4: 1.

10. A process as claimed in claim 6 wherein said CuO is at least partlyconverted into CuS in said reaction zone, said CuS together withunreacted CuO is continuously removed from the reaction zone and thenoxidized to reform CuO, and trithiane is again continuously conductedthrough said reaction zone in admixture with said reformed CH0 and air.

References Cited Girard, Comptes Rendus de lAcademie des Sciences, vol.70, pp. 625-629 (1870).

LEON ZITVER, Primary Examiner.

R. H. LILES, Assistant Examiner.

1. A PROCESS FOR THE PRODUCTION OF FORMALDEHYDE FROM TRITHIANE WHICHCOMPRISES CONTACTING SAID TRITHIANE WITH CUO AND AIR AT A TEMPERATUREBETWEEN ABOUT 190*C. AND 250*C. AND CONDENSING FORMALDHYDE FROM THEEFFLUENT GASEOUS REACTION PRODUCT.